This invention relates to vehicle occupant weight sensing with fluid filled seat bladders, and more particularly method of relieving initial stress in the fluid filled seat bladders.
Vehicle occupant weight detection has been used as a means of identifying occupant presence and type (i.e., adult or child) in connection with the deployment of inflatable restraints in a severe crash event. A particularly effective technique for detecting occupant weight is disclosed in the U.S. Pat. No. 5,987,370 to Murphy et al., assigned to the assignee of the present invention, and incorporated by reference herein. In Murphy et al., a fluid filled elastomeric bladder is installed in or under the foam cushion of a vehicle seat bottom, and a pressure sensor coupled to an exit port of the bladder senses the pressure of the fluid as a measure of occupant weight applied to the seat bottom. See also, the related U.S. Pat. Nos. 6,058,341 and 6,101,436, and the allowed U.S. Pat. No. 6,138,067, which likewise are assigned to the assignee of the present invention and likewise are incorporated herein by reference.
In developing the aforementioned bladder-based weight sensing, we have observed that stretching of the bladder material during fluid filling creates an initial stress in the bladder material that is gradually relieved over time. Unfortunately, such stress produces error in the occupant weight indication that cannot be not easily compensated by an offset or the like.
The present invention is directed to an improved method of producing a fluid filled elastomeric bladder for occupant weight sensing in which an initial stress due to stretching of the bladder material is quickly relieved by annealing the bladder material prior to its installation in a vehicle seat. In a preferred embodiment, the initial stress is relieved by immersing the fluid-filled bladders in heated water for a time period on the order of 60 seconds or more. Alternatively, the bladder material may be heated with steam or radiant heat, by injecting heated fluid into the bladder, or by injecting heated air into an empty bladder before fluid injection. In each case the initial stress is quickly relieved, substantially eliminating a potential source of error in the sensed weight.